The present invention relates to a novel hole transporting material.
With development of the electronics field there is a demand for a material capable of efficiently transporting either electrons or holes in fabricating various electronics devices.
Hole transporting materials which carry only holes in high efficiency are now in wide use as photoreceptor in electrophotography, and their application to organic electroluminescent devices is expected.
For example, an organic photoconductor (hereinafter referred to simply as "OPC") has recently attracted attention of many concerns in quick tempo as an organic photoreceptor mainly in its application to electrophotography. When used as a photoreceptor, OPC plays the role as a carrier transporting material.
OPC possesses various features, such as advantage in processability, easy to constitute a device, good electric charge accepting and holding properties, and advantage in cost. Therefore, researches are being made energetically not only about using OPC as a substitute for photoreceptor using inorganic materials but also about the development of a organic photoreceptor of OPC having a new performance.
Heretofore, various organic photoconductive compounds have been known as OPC's. Typical examples include various low molecular weight organic compounds such as triphenylamine derivatives, pyrazoline derivatives, oxadiazole derivatives, indoline derivatives and carbazole derivatives. These various OPC's per se are low molecular compounds, so unmoldable, and a large area which is required for a photoreceptor cannot be attained by the use of them. For example, therefore, such OPC's are mixed and dispersed in a transparent high polymer such as polycarbonate as a matrix polymer to prepare OPC photoreceptor.
Therefore, a restriction is placed on the OPC content in the matrix polymer. At a high OPC concentration, a uniform dispersibility of OPC in the matrix is deteriorated, so it has been difficult to use OPC at a high concentration. For this reason, a limit is also reached in the carrier transporting ability in a OPC photoreceptor, that is, it is impossible to attain a high carrier mobility. Under the circumstances it has been desired to develop an OPC having superior photoconductivity.
An electroluminescent device (ELD) has long been noted as a thin large-area light source. With the recent spread and reduction in size and weight of office automation (OA) devices, a liquid crystal display has come to be widely adopted as a display device, and now there is an increasing expectation for ELD as a back light for the liquid crystal display.
ELD, which is a thin, flat shape light emitting device of a light weight, possesses such features as no flicker on the screen, a wide visual angle and a low power consumption. However, since the ELD which has heretofore been put to practical use is operated by a light emitting mechanism called a collision excitation type, it is necessary to use a high driving AC voltage of 200 volts or so; besides, the light emission efficiency is low and the luminescence is also low.
C.W. Tang et al. [Appl. Phys. Lett. 51(12), 913 (1987)] manufactured for trial an organic thin film ELD of an injection type different from the conventional light emitting mechanism, and showed that the drawbacks of the conventional AC voltage driven, collision excitation type ELD. An injection type ELD is basically of a sandwich structure wherein a light emitting layer is sandwiched in between an electron injection electrode and a hole injection electrode. But, according to the ELD developed by C.W. Tang et al., an arylamine-based organic hole transporting layer is formed by vapor deposition between the light emitting layer and the hole injection electrode, and as the electron injection electrode there is used a metal having a small work function such as, for example, magnesium, whereby they showed that the ELD could be driven at a low DC voltage of several ten volts or so and that a high luminance light emission comparable to that of a light emitting diode could be attained. It is apparent that an important component for fabricating a low DC voltage driven, organic thin film ELD of high luminance is an organic hole transporting layer. In order to obtain an ELD of a practical level and a higher performance it is desired to obtain a hole transporting material which can be easily made uniform and small in film thickness. Particularly, for fabricating an ELD of a large area on an industrial scale it is necessary to form a thin hole transporting film having a uniform thickness, free of defects such as pin-holes and high in reliability. But it cannot be said that the conventional means such as vapor deposition satisfy those requirements.
Thus, the conventional hole transporting materials involve various problems.
It is an object of the present invention to solve the . above-mentioned problems of the prior art.
It is another object of the present invention to provide a novel hole transporting material having extremely superior hole transporting ability and processability, that is, capable of overcoming all of the problems of the conventional OPC's and hole moving materials typified by hole transporting materials for ELD.